diff --git a/src/renderer/src/components/editor/editor-panel-render-model.ts b/src/renderer/src/components/editor/editor-panel-render-model.ts
index 0ccf9053c3f..144264b5753 100644
--- a/src/renderer/src/components/editor/editor-panel-render-model.ts
+++ b/src/renderer/src/components/editor/editor-panel-render-model.ts
@@ -133,7 +133,10 @@ export function getEditorPanelRenderModel({
   const richModeEligibility = canRenderInlineMarkdown
     ? getCachedMarkdownRichModeEligibility({
         content: inlineMarkdownContent,
-        sizeOverridden: markdownRichModeSizeOverridden
+        sizeOverridden: markdownRichModeSizeOverridden,
+        // Source mode only needs a conservative affordance decision. The full
+        // HTML round trip is reserved for rich rendering so typing stays cheap.
+        validateHtmlRoundTrip: mdViewMode === 'rich'
       })
     : null
   const richModeUnsupportedMessage = richModeEligibility?.unsupportedMessage ?? null
diff --git a/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.test.ts b/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.test.ts
index f94f0a09cd2..6510c11eaf9 100644
--- a/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.test.ts
+++ b/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.test.ts
@@ -1,5 +1,7 @@
 // @vitest-environment happy-dom
-import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import * as htmlValidation from './markdown-rich-html-validation'
+import * as roundTrip from './markdown-round-trip'
 import { i18n } from '@/i18n/i18n'
 import { getMarkdownRichModeEligibility } from './markdown-rich-mode'
 import {
@@ -73,6 +75,21 @@ describe('getCachedMarkdownRichModeEligibility', () => {
     )
   })
 
+  it('supports a conservative source-mode decision without HTML round trips', () => {
+    const htmlValidationSpy = vi.spyOn(htmlValidation, 'getRichMarkdownHtmlValidationOutput')
+    const roundTripSpy = vi.spyOn(roundTrip, 'getRichMarkdownRoundTripOutput')
+
+    const result = getCachedMarkdownRichModeEligibility({
+      content: '<span>edited</span>\n',
+      sizeOverridden: false,
+      validateHtmlRoundTrip: false
+    })
+
+    expect(result.unsupportedMessage).not.toBeNull()
+    expect(htmlValidationSpy).not.toHaveBeenCalled()
+    expect(roundTripSpy).not.toHaveBeenCalled()
+  })
+
   it('stays correct once the corpus exceeds the cache capacity', () => {
     const expected = CORPUS.map(({ content }) =>
       getMarkdownRichModeEligibility({ content, sizeOverridden: false })
diff --git a/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.ts b/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.ts
index 9d54df171fa..0528f2a7489 100644
--- a/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.ts
+++ b/src/renderer/src/components/editor/markdown-rich-mode-eligibility-cache.ts
@@ -8,6 +8,7 @@ import {
 type EligibilityCacheEntry = {
   content: string
   sizeOverridden: boolean
+  validateHtmlRoundTrip: boolean
   decision: MarkdownRichModeEligibilityDecision
 }
 
@@ -36,6 +37,7 @@ const entries: EligibilityCacheEntry[] = []
 export function getCachedMarkdownRichModeEligibility(params: {
   content: string
   sizeOverridden: boolean
+  validateHtmlRoundTrip?: boolean
 }): MarkdownRichModeEligibility {
   const decision = getCachedMarkdownRichModeEligibilityDecision(params)
   return {
@@ -47,11 +49,16 @@ export function getCachedMarkdownRichModeEligibility(params: {
 function getCachedMarkdownRichModeEligibilityDecision(params: {
   content: string
   sizeOverridden: boolean
+  validateHtmlRoundTrip?: boolean
 }): MarkdownRichModeEligibilityDecision {
-  const { content, sizeOverridden } = params
+  const { content, sizeOverridden, validateHtmlRoundTrip = true } = params
   for (let index = 0; index < entries.length; index += 1) {
     const entry = entries[index]
-    if (entry.sizeOverridden !== sizeOverridden || entry.content !== content) {
+    if (
+      entry.sizeOverridden !== sizeOverridden ||
+      entry.validateHtmlRoundTrip !== validateHtmlRoundTrip ||
+      entry.content !== content
+    ) {
       continue
     }
     if (index > 0) {
@@ -61,8 +68,12 @@ function getCachedMarkdownRichModeEligibilityDecision(params: {
     return entry.decision
   }
 
-  const decision = getMarkdownRichModeEligibilityDecision({ content, sizeOverridden })
-  entries.unshift({ content, sizeOverridden, decision })
+  const decision = getMarkdownRichModeEligibilityDecision({
+    content,
+    sizeOverridden,
+    validateHtmlRoundTrip
+  })
+  entries.unshift({ content, sizeOverridden, validateHtmlRoundTrip, decision })
   if (entries.length > MAX_ENTRIES) {
     entries.length = MAX_ENTRIES
   }
diff --git a/src/renderer/src/components/editor/markdown-rich-mode.ts b/src/renderer/src/components/editor/markdown-rich-mode.ts
index 82a86171a3b..8644362ec55 100644
--- a/src/renderer/src/components/editor/markdown-rich-mode.ts
+++ b/src/renderer/src/components/editor/markdown-rich-mode.ts
@@ -112,7 +112,8 @@ export function resolveMarkdownRichModeUnsupportedMessage(
 }
 
 export function getMarkdownRichModeUnsupportedReason(
-  content: string
+  content: string,
+  { validateHtmlRoundTrip = true }: { validateHtmlRoundTrip?: boolean } = {}
 ): MarkdownRichModeUnsupportedReason | null {
   // Why: front-matter is handled externally — stripped before the rich editor
   // sees the content and displayed as a read-only block. Only the body needs
@@ -145,6 +146,9 @@ export function getMarkdownRichModeUnsupportedReason(
   }
 
   if (hasHtml) {
+    if (!validateHtmlRoundTrip) {
+      return htmlMatcher!.reason
+    }
     // The source codec recognizes multiline code spans that the cheap scan can misclassify.
     const htmlOutput = getRichMarkdownHtmlValidationOutput(body)
     if (htmlOutput && preservesEmbeddedHtml(body, htmlOutput)) {
@@ -163,14 +167,16 @@ export function getMarkdownRichModeUnsupportedReason(
 
 export function getMarkdownRichModeEligibilityDecision({
   content,
-  sizeOverridden
+  sizeOverridden,
+  validateHtmlRoundTrip = true
 }: {
   content: string
   sizeOverridden: boolean
+  validateHtmlRoundTrip?: boolean
 }): MarkdownRichModeEligibilityDecision {
   return {
     exceedsSizeLimit: !sizeOverridden && exceedsMarkdownRichModeSizeLimit(content),
-    unsupportedReason: getMarkdownRichModeUnsupportedReason(content)
+    unsupportedReason: getMarkdownRichModeUnsupportedReason(content, { validateHtmlRoundTrip })
   }
 }